The present invention relates generally to the field of drying systems for agricultural grains and other particulate materials. More particularly, the present invention relates to a drying control system and method which uses a moisture sensor. The present invention is specifically described with respect to the drying of agricultural grain, but the principles involved are also applicable to other particulate materials.
A grain dryer typically consists of a bin or chamber with an apertured floor. Grain is placed in the bin and warm dry air is forced up through the apertured floor. The air circulates around the grain particles, working its way up through the grain in the bin. In doing so, the air warms the grain and absorbs some of its moisture, and in turn, the air is cooled and becomes moisture laden. In this manner, drying proceeds upwardly in zones through the drying bin until the desired lower level of moisture content is attained. Periodically, as the grain is being dried, the warmest and driest layers from the bottom of the drying bin are drawn off or removed for storage or shipment. The method of removal is usually powered augers, namely, sweep augers, discharge augers and transfer augers. The speed and/or continuity of operation of the sweep and discharge augers determines the rate at which the grain or other particulate material moves through the drying bin, and, inversely, the length of time during which the grain is exposed to the drying action of the warm dry air. A transfer auger transfers grain from the discharge auger to a storage bin.
Because the drying process can proceed at different rates, depending upon the moisture content of the grain, ambient air temperature and humidity, and the intensity of the applied heat, it is necessary to provide some type of control system. Generally, it is convenient to allow the air heating and circulating equipment to operate according to its optimum design characteristics, and to control the overall drying by controlling the removal rate of the dried grain from the bottom of the bin. This, in turn, is done by controlling the sweep and discharge augers periodically according to a preset timer, intermittently according to sensed temperature or moisture, or by a combination of both.
The prior art has many types of sensing systems for sensing humidity or temperature of the grain or air at a selected zone. One type of system uses a sensing element placed at a point around the periphery of the drying bin at a preselected elevation above the floor. However, this type of system has certain inherent disadvantages because its operation depends on the assumption that uniform drying occurs at equal elevations above the floor. However, in practice there may be wet spots or zones which may be missed by this type of sensor. Other types of sensors are mounted at the discharge auger from the bin for sampling the moisture content or temperature of the grain being discharged or air escaping therefrom. Often in these types of systems, the motor for the sweep and discharge augers is started periodically by a timer, then remains in motion until the temperature or wetness of the grain can be sampled at the discharge. If the grain has too high of a moisture content, the discharge mechanism is stopped to await another predetermined time interval while the drying apparatus continues in operation.
U.S. Pat. No. 3,714,718 discloses a moisture sensor near the grain discharge outlet and seems to imply that it does not require periodic sampling, since it measures the moisture in the air which continuously escapes past the moisture sensor. In addition to other problems, there is the problem of variations in signal due to numerous environmental and system factors thereby reducing the accuracy of the system. Moreover, there is the problem that the sensor senses the moisture level of the air and not the grain itself, which provides a less accurate measurement of the moisture content in the grain, than sensing the grain itself. Additionally, there is a potential problem of rapid on/off switching.
U.S. Pat. No. 4,599,809 also disclosed a grain moisture sensing system. This system receives a grain sample from the grain unloaders and conveys the sample to a capacitive sample cell where a meter senses the moisture content as a function of the dielectric constant of the sample in the cell. Some of the disadvantages of this system are that the moisture of the grain is only periodically sensed and a separate sampling cell is needed to do so.
Commonly assigned patent, U.S. Pat. No. 4,152,840, hereby incorporated by reference, disclosed a sensing system wherein a predry sensor is mounted inside the grain drying bin and a discharge grain temperature sensor is mounted in the discharge auger to measure the temperature of the grain in the discharge auger. While this approach offers many advantages over the prior art, the present invention offers even further advantages over existing grain drying systems.